The measurement of friction force between two flat surfaces is routinely performed using an object of known weight having a flat bottom surface (a "sled"), and a larger, flat, horizontal surface (the "table"). The sled rests on the table with the friction surfaces to be evaluated in contact. A shear force is applied which tends to cause sled and table to slide with respect to one another. The force required to initiate sliding is called the static friction force, the force required to sustain sliding, the kinetic friction force.
While elementary in concept, the actual test is often problematic. It is not uncommon to find that when friction force is evaluated by different test technicians, or on different instruments, significantly different results are obtained. Recent research has shown that friction force can be changed by what appear to be minor differences in the way in which the test is performed, and by inadvertent contamination of the test-pieces by the operator. These findings place stringent new demands on test standards, procedures and instruments to control relevant variables and to prevent the operator from influencing the test results.